1. Field of the Invention
The present invention relates to an optical communication system. More particularly, the present invention relates to an optical switching device provided at the optical communication system to variably set the travel path of optical signals.
2. Description of the Related Art
Recently, there has been shown the practical use of a wavelength division multiplexing (WDM) technique adapted to transmit a plurality of channels having different wavelengths through a single-core optical fiber. This technique has made it possible to transmit large quantities of data at high speed. Also, optical switching for optically setting travel paths of optical signals has been possible in accordance with development of optical element manufacturing techniques. As a result, construction of a WDM type optical communication network has become possible.
For such a WDM type optical communication network, it would be best to use a mesh type communication network. The mesh type communication network uses an optical switching device, such as an optical cross-connect device, for operation. The optical switching device serves to transmit an optical signal, received through an input port, to a one predetermined output port selected from a plurality of output ports. In the case where the optical switching device receives an optical signal multiplexed with a plurality of channels, there should be control such that each channel is independently switched without being influenced by other channels.
Typically, conventional optical switching devices include a plurality of switches, as shown in FIGS. 1 and 2. In the cases of FIGS. 1 and 2, the switches 111 to 123 are illustrated as being arranged in a matrix array, for explanatory purposes. Also, each switch is designated by its row number and its column number. For example, the switch 111, which is arranged on the first row and the first column, is referred to as “SW11 switch”, and the switch 122, which is arranged on the second row and the second column is referred to as “SW22 switch”. Each of the switches 111 to 123 has a plurality of ports. Where it is assumed that one switch is designated by a reference numeral “###”, its N-th port is designated by “N” in the drawings while being designated, in the following description, by a reference numeral “###N”.
FIG. 1 is a block diagram illustrating an example of a conventional 4×4 optical switching device. This optical switching device has first through fourth input ports IN1 to IN4, and first through fourth output ports OUT1 to OUT4. The optical switching device includes 6 switches 111 to 123 arranged to have a 2×3 matrix array. Each of the switches 111 to 123 is connected to another switch arranged on the same row in the matrix array while being connected to another switch arranged on a row adjacent thereto in the matrix array. Each of the switches 111 to 123 has first and second ports at an input side thereof, and third and fourth ports at an output side thereof. Each switch is switched between a “cross” state and a “bar” state in accordance with a control signal from a control unit (not shown).
For example, when the SW11 switch 111 is in a bar state, it outputs an optical signal, inputted thereto at its first port 1111 (designating switch 111, port 1 or 1111), at its third port 1113 (switch 111, port 3, referred to as 1113), while outputting an optical signal, inputted thereto at its second port 1112, at its fourth port 1114. On the other hand, when the SW11 switch 111 is in a cross state, it outputs an optical signal, inputted thereto at its first port 1111, at its fourth port 1114, while outputting an optical signal, inputted thereto at its second port 1112, at its third port 1113. The control unit controls the switches 111 to 123 in order to output an optical signal, inputted to an optional input port of the optical switching device, to an associated output port.
For example, the case in which an optical signal is input to the first input port IN1 is required to be output to the second output port OUT2 as will be described hereinafter. In this case, the control unit maintains the SW11 switch 111 in its cross state, while maintaining the SW22 switch 122 and SW13 switch 113 in their bar state.
Accordingly, it can be seen that the optical signal inputted to the first input port IN1, that is, the first port 1111 of the SW11 switch 111, is outputted to the fourth port 1114 of the SW11 switch 111 which, in turn, applies the optical signal to the first port 1221 of the SW22 switch 122. The optical signal inputted to the first port 1221 of the SW22 switch 122 is outputted to the third port 1223 of the SW22 switch 122 which, in turn, applies the optical signal to the second port 1132 of the SW13 switch 113. The optical signal inputted to the second port 1132 of the SW13 switch 113 is outputted to the third port 1133 of the SW13 switch 113. Thus, the optical signal is outputted to the second output port OUT2.
FIG. 2 is a block diagram illustrating an example of a conventional 8×8 optical switching device. This optical switching device has first through eighth input ports IN1 to IN8, and first through eighth output ports OUT1 to OUT8. The optical switching device includes 16 switches 211 to 244 arranged to have a 4×4 matrix array. Each of the switches 211 to 244 is connected to another switch arranged on the same row in the matrix array while being connected to another switch arranged on a row adjacent thereto or a row adjacent to the adjacent row in the matrix array. Each of the switches 211 to 244 has first and second ports at an input side thereof, and third and fourth ports at an output side thereof. Each switch is switched between a “cross” state and a “bar” state in accordance with a control signal from a control unit (not shown). The control unit controls the switches 211 to 244 in order to output an optical signal, inputted to an optional input port of the optical switching device, to an associated output port.
For example, for the case in which an optical signal is input to the first input port IN1 has to be outputted to the fifth output port OUT5 will be described hereinafter. In this case, the control unit maintains the SW12 switch 212 and SW33 switch 233 in their cross state, while maintaining the SW11 switch 211 and SW34 switch 234 in their bar state.
Accordingly, the optical signal input to the first input port IN1, that is, the first port 2111 of the SW11 switch 211, is output to the third port 2113 of the SW11 switch 211 which, in turn, applies the optical signal to the first port 2121 of the SW12 switch 212. The optical signal input to the first port 2121 of the SW12 switch 212 is output to the fourth port 2124 of the SW12 switch 212 which, in turn, applies the optical signal to the second port 2332 of the SW33 switch 233. The optical signal input to the second port 2332 of the SW33 switch 233 is output to the third port 2333 of the SW33 switch 233. In turn, the switch 233 applies the optical signal to the first port 2341 of the SW34 switch 234. The optical signal input to the first port 2341 of the SW34 switch 234 is output to the third port 2343 of the SW34 switch 234. Thus, the optical signal is output to the fifth output port OUT5.
As described above, the conventional optical switching devices implement an N×N optical switching device having N input ports and N output ports by using 2×2 switches as basic constitutive elements. However, where such an N×N optical switching device having the above mentioned arrangement is implemented, it is necessary to use a number of 2×2 optical switching elements. For example, where the 4×4 optical switching device shown in FIG. 1 is implemented, six 2×2 switches should be used. In other words, the conventional optical switching devices have a problem in that they have a complex configuration because a uniform connection method is used for connection of 2×2 switches, without taking into consideration the characteristics of those 2×2 switches, so that a number of 2×2 switches should be used to implement a desired N×N optical switching device.
Furthermore, the conventional optical switching devices involve high manufacturing costs because each 2×2 switch is implemented by a plurality of optical elements or an expensive integrated element.